Change format_text to format_explanation

Also fixes issues with it.

CHANGELOG

@@ -89,8 +89,8 @@ Version 0.4: Released xxx
 * Added ``explain`` transform: it allows you to set the "explain" flag
   which gives you an explanation of how the score was calculated.
 
-  I also added ``elasticutils.utils.format_text`` which formats the
-  resulting explanation text into something slightly more
+  I also added ``elasticutils.utils.format_elasticutils`` which formats
+  the resulting explanation text into something slightly more
   readable. But it's likely this will change in the future.
 
 * Added ``boost`` transform: it allows you to do query-time field

elasticutils/utils.py

@@ -1,9 +1,11 @@
-def format_text(explanation, indent='  ', indent_level=0):
+def format_explanation(explanation, indent='  ', indent_level=0):
     """Return explanation in an easier to read format
 
     Easier to read for me, at least.
 
     """
+    if not explanation:
+        return ''
 
     # Note: This is probably a crap implementation, but it's an
     # interesting starting point for a better formatter.
@@ -13,7 +15,7 @@ def format_text(explanation, indent='  ', indent_level=0):
 
     if 'details' in explanation:
         details = '\n'.join(
-            [format_text(subtree, indent, indent_level + 1)
+            [format_explanation(subtree, indent, indent_level + 1)
              for subtree in explanation['details']])
         return line + '\n' + details